Step motor damping method and apparatus

ABSTRACT

A step motor damping apparatus and method which provide electrical damping of the step motor motion. A damping control signal is derived from the electromotive force induced in certain stator pole windings of the step motor by the moving motor armature. A damping force on the armature is produced by energizing the same or other motor poles in response to the damping signal.

United States Patent 11 1 Rich,

1 1 STEPMOTOR DAMPING NIETHOD AND APPARATUS [75] Inventor: Leonard G.Rich, West Hartford,

- v Conn.

[73 Assignee: The Gerber Scientific Instrument Company, Hartford, Conn.

22 Filed: Apr. 8, 1971 i 21 Appl,No.: 132,355

[58] Field of Search ..318/138, 254, 696,

[56] References Cited [451 Apr. 10, 1973 Primary Examiner-G. R. SimmonsAttorney-McCormick, Paulding 81. Huber [57] ABSTRACT A step motordamping apparatus and method which provide electrical damping of thestep motor motion. A damping control signal is derived from theelectromotive force induced in certain stator pole windings of the stepmotor by the moving motor armature. A damping force on the armature isproduced by energizing the same or other motor poles in response UNITEDSTATES PATENTS to the damping slgnal- 3,423,658 1/1969 Barrus ..318/69615 Claims, 5 Drawing Figures 4e m/vm sw/rcHr AND zwps.

INPUT OUTPUT SELECTOR SELECTOR 56 CIRCUIT CIRCUIT DRIVE}? 1 T LOG/CPATENTEU 0W 3. 727. 121

SHEET 1 OF 2 INVE/VTOR LEONARD GR/CH Almrne'y PATENTEDAPR] 01375 SHEET 2OF 2 Wm QV RSQQC mmkuuimm RDOCHQ MQQQQ Q5 .WIMIURRAM STEP MOTOR DAMPINGMETHOD AND APPARATUS BACKGROUND OF THE INVENTION This invention relatesto the field of electric motors and, more particularly; is concernedwith motors of the impulse type, commonly referred to as steppingmotors, wherein the armature of the motor is moved in a number ofdiscrete displacements or steps as a result of a corresponding number ofdiscrete changes in the electrical energization scheme of the motor polewindings.

In both rotary and flat or linear step motors of the type to which thepresent invention is directed, the motor is composed generally of amovable armature adapted for connection to a mechanical load and astator assembly or base platen having a number of magnetically permeablepoles. A suitable electrical drive logic or control unit which forms apart of the stepping motor drive system operates to excite the windingsof the motor poles in such a manner that a magnetic field is createdwhich produces a magnetomotive force or torque which urges the motorarmature to assume a mechanical position in line with the resultantmagnetic field. The control unit changes the excitation state orcondition of the pole windings in a phased sequence to cause themagnetic field to move in a stepby-step fashion and as the magneticfield so moves, the motor armature follows with a correspondingstep-by-step movement.

Depending upon the number of leads or windings available as separateinputs, the motor may require three, four, five or more changes in thephased excitation sequence to achieve a complete cycle of electricalexcitation, generally defined as 360 of electrical rotation. The actualmechanical movement of the motor armature produced by 360 electricaldegrees of rotation may not correspond to 360 of mechanical rotation inrotary motors, but depends instead upon the geometry and design of theparticular motor. In both rotary step motors and flat or linear stepsmotors, the latter of which are basically rotary'motors of infinitearmature radius, some definite number of discrete steps and a definiteamount of mechanical movement is always prevalent or directly associatedwith each cycle of the electrical excitation. In a simple rotary motor,one full rotation of the armature may result from 360 electrical degreesof rotation with four steps being required to achieve such rotation. Inother rotary motors of a slightly more complex geometry, 200 steps ormore may be required to obtain 360 mechanical degrees of rotation of thearmature. In the latter motors, a complete 360 cycle of mechanicalrotation may be accompanied by many full cycles of electricalexcitation.

In both flat and'rotary step motors, the motor armature is urged to'makean instantaneous motion toward the next magnetically stable positionafter each step change in the excitation of the pole windings. Assumingno static load on the motor armature and no damping, the motor armatureresponds to the step change by accelerating toward the newly commandedposition until that position is reached. At the new position, the torqueon the armature due to the magnetic field is reduced to zero; however,the kinetic energy acquired by the armature during acceleration towardthe new magnetically stable position carries the annature past the newposition and a torque reversal is automatically generated to return thearmature toward the commanded position. The reversal of the torque onthe armature continues to retard the overshooting motion of the armatureuntil the kinetic energy is entirely converted to potential energy inthe magnetic field at the new position. At this point, the armature isaccelerated back toward the commanded position and again overshoots theposition but in the opposite direction. The oscillatory motion of thearmature about the commanded position continues until the energy of thesystem has been dissipated in the same manner as in the classicalmass-spring system.

The oscillation and overshoot of the armature at the newly commandedposition is ordinarily not desirable and may degrade the output of thestep motor, particularly at low motor speeds, to a point which makes itunacceptable for functions, such as controlling the movement of acutting tool or plotting stylus in programs which include low velocityroutines in one or more of its coordinate directions of movement.

Accordingly, it is desirable to have a method and apparatus for dampingthe movement of a step motor armature to eliminate the undesirableoscillatory overshoot normally associated with each discretedisplacement of such armature.

SUMMARY The present invention resides in a method and apparatus fordamping the oscillatory overshoot movements of a step motor armature.The invention employs a sensing means incorporating the motor poles andwindings, for detecting the oscillatory overshoot movement of the stepmotor armature, and force generating means connected between the sensingmeans and the motor armature for imposing a dampingforce on the armaturein response to the movement detected by the sensing means. The sensingmeans detects the change in magnetic flux in the motor poles caused bythe oscillatory overshoot movements of the armature, which flux changeis manifested as an electromotive force or voltage induced in the motorpole windings. This voltage is sequentially detected in selected polewindings which due to their momentary arrangement relative to thearmature have passing therethrough a magnetic flux which issubstantially linearly related to the displacement of the armature fromits commanded position with the result that the detected voltage signalis substantially a sine wave in phase with the velocity of the armatureas it undergoes its oscillatory overshoot movement. The force generatingmeans responds to the detected voltage signal and produces a relateddamping force on the motor armature. The force generating means in apreferred embodiment includes sequentially selected pole windings whichare excited by the detected voltage signal so that a magnetomotivedamping force is imposed on the armature to damp the armature movements.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of theprincipal operating components of a rotary step motor.

FIG. 2 is a schematic representation of a portion of a rotary step motorwhich has been flattened into a linear configuration for purposes ofunderstanding the operation of a step motor.

FIG. 3 is a graph showing the magnetic flux passing through a motor poleand its associated armature pole as a function of the deviation of thestep motor annature from an aligned or commanded position relative tothe motor pole.

FIG. 4 is a diagram showing the components'in a step motor drive systemembodying the present invention.

FIG. 5 is a' diagram showing the components of another step motor drivesystem embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the principalcomponents of a rotary step motor, generally designated 10, which may bedamped according to the method and apparatus of the present invention.The principal components of the motor include a rotor or rotary armaturel2 and stator poles 14, 14. It should be understood that a plurality ofstator poles 14, 14 are'distributed at evenly spaced intervals aroundthe armature 12; however, for the purposes of clarity, only two suchpoles are shown in FIG. 1. In a complete motor, the rotary armature 12is supported by suitable bearings along its axis of rotation'l6 within acasingto which the stator poles 14 are fixedly attached. Sincethe'detailed construction of the motor casing and the bearing structureis not essential to a clear understanding of the present invention, suchdetails have been omitted and only the principal com ponents whichproduce the stepping motion of the rotor are shown.

The rotary armature 12 is the movable element of the rotary step motor10 and includes two cylindrical end members 18 and 20 and a coaxialpermanent magnet 22 interconnecting the members 18 and 20. The

FIG. 2. To identify the poles 14, 14 and their subparts,

the same reference numeral is given to corresponding parts of the polesand a letter subscript is added where necessary to distinguish theindividual parts of one pole from another. It will be noted thatalthough five poles 14 are shown, to indicate continuity, the pole onthe right in FIG. 2 is shown in phantom and designated 14a, the same asthe pole on the left, as would be the case where a total of four suchpoles are evenly distributed at 90 intervals about the cylindricalmember 18. While in normal practice'the number of poles is much largerthan four and depends upon the size of the poles and circumference ofthe member 18, only four poles are north and south poles of the magnet22 are indicated by the letters N and S," and confront axial faces ofthe members 18-and 20 respectively. The cylindrical members l8 and 20and the magnet 22 form a composite body which is symmetrically disposedabout axis 16.

Each of the end members 18 and 20 has a similar construction and isformed from a magnetically permeable material such as iron or steelalloy. The circumferential surfaces of the members 18 and 20 include aplurality of axially extending parallel grooves which define a numberofteeth 21, 21 spaced evenly about the circumferential surface ofthe-members. As a result of the magnet 22 the end member 18 is chargedmagnetically north and the end member 20 is charged magnetically south.

Each stator pole 14 is basically an electromagnet composed of amagnetically permeable core 24 and an excitation winding 26. Theopposite ends of the core 24 each have three axially extending teeth 28,28 which are of circumferential width and spacing equal to thecircumferential width and spacing of the rotor teeth 21, 21. The radialdistance between the teeth 28, 28 of the pole 14 and the adjacentrotorteeth 21, 21 is relatively small in comparison to thecircumferential separation between the teeth. In this manner, a completemagnetic flux path can be traced through the armature l2 and each of thepoles 14, 14.

In order to more clearly understand the operation of the step motor 10,the end member, 18 and the mating end portions of a plurality of thestator poles 14, 14 have been distorted into a flattened configurationin needed to explain the step-by-step sequencing of the rotor.

The rotary armature 12 is shown in FIG. 2 at a position in which threeteeth 21, 21 of member 18 are aligned or in registry with the threeteeth 28, 28 of pole 14a. A comparison of the relationship between thearmature teeth and the pole teeth of the other poles 14b, 14c and 14ddiscloses that the different poles are spaced so that the teeth on nomore than one pole in the set of four are aligned with the teeth on themember '18 at any given time. The teeth of the pole 14c are locateddirectly opposite the spaces between the teeth of member 18 orsymmetrically offset from the adjacent teeth on the member 18. The teethon pole 14b with respect to the teeth of member 18 are in turnasymmetrically offset from the teeth on member 18 or halfway between thealigned position of the teeth on pole 14a and the symmetrically offsetposition of the teeth on pole 14c. The teeth on pole 14d with respect tothe teeth on member 18 are asymmetrically offset from the teeth onmember 18 or halfway between the symmetrically offset position of theteeth on pole 14c and the aligned position of the teethon pole 14a. Withthe relationship shown, movement of member 18 to the right in FIG. 2sequentially brings the teeth on the member 18 and the teeth on poles14b, 14c and 14d into registry in that'order and'further movement-againreestablishes registry with the teeth on pole 14 a at which point thecycle can be repeated.

In one mode of operating the step motor represented in FIG. 2, the polesare excited in what is termed a unipolar manner. Foe example,'in astatic condition with the components shown as in FIG. 2, the winding 26aof pole 14a is energized producing a magnetic south pole at the poleface adjacent the member 18 and a magnetic north pole at the pole face(not shown in FIG. 2) adjacent the other end member 20. Such excitationin the absence of external loads holds the teeth ofarmature l2 and pole14a in alignment since this stable magnetic-mechanical configurationcorresponds to the minimum energy condition with maximum flux andmaximum permeability.

If the energization of pole 14a is terminated and pole 14b is energizedto create a magnetic south pole adjacent the member 18 and a magneticnorth pole adjacent the member 20, the armature 12 is caused to index orstep by a half-tooth width so that the armature teeth and the teeth onpole 14b become aligned. Subsequent de-energization of pole 14b andenergization of pole 14c in the same manner brings the teeth of armaturel2 and the pole into alignment since the indexing movement produced bythe excitation of pole 14b established the asymmetric offset of thearmature teeth and the teeth of pole 140. In a still further step in thesequence, de-energization of pole 14c and energization of pole 14dproduces a further indexing and alignment of the armature teeth and theteeth of pole 14d. Subsequent deenergization of pole 14d andenergization of pole 14a reestablishes alignment of the armature teethand pole 14a and completes the electrical energization cycle anddisplacement of the armature by one full increment equal to the distancebetween corresponding edges of adjacent teeth. Accordingly, the foursteps of the electrical cycle produce l/n of a complete mechanicalrevolution of the armature where n equals the number of teeth on themember 18. It is common practice in the step motor art to employ stepmotor designs which require 200 steps per revolution of the armature bysequential excitation of the stator windings.

The operating mode described above is termed unipolar excitation sinceonly one pole is energized at a time and always with the same polarity;however, it is also possible to excite the stator poles in pairs. Forexample, in addition to exciting pole 14a in FIG. 2 to produce a southpole adjacent member 18 pole 14c can be simultaneously energized toproduce a north pole which tends to establish a mutual repulsion betweenthe teeth of pole 14c and the adjacent teeth of member 18. Since theteeth of pole 140 are equidistant from or symmetrically offset from theadjacent teeth of member 18, again the minimum energy condition and astable magnetic-mechanical configuration are present. Such bi-polarexcitation requires alternate reversals of the excitation currents togenerate both north and south poles through the same windings and iscommonly employed in the step motor art. The damping control of thepresent invention can be employed with either the uni-polar or bi-polarexcitation schemes.

A principal feature of the present invention resides in sensing meanswhich utilizes the windings of the motor poles for detecting oscillatoryovershoot movement of the step motor armature.

As described above, the indexing of a motor armature betweenmagnetically stable positions is accompanied by an oscillation of thearmature about the motor pole at the newly commanded position. Voltagesinduced in the non-excited motor pole windings by the oscillatingarmature bear a unique relationship to the armature velocity and may beutilized as electrical signals for damping the armature motions.

FIG. 3 discloses a plot showing the flux d: between the armature teethand the excited stator pole teeth of opposite polarity as a function ofarmature displacement on opposite sides of the magnetically stable,commanded or zero position of the armature adjacent the stator pole. The-X coordinates represent positions on one side of the zero positionwhile the +X coordinates represent positionson the other side. As thearmature oscillates about the zero position, a voltage or back EMF(electromotive force) is induced in each motor pole winding and thevalue of the EMF is equal to N(d/dt), where 4) is the flux and N is thenumber of linked turns forming the winding.

} It is clear from inspection of FIG. 3 that the variation of thevoltage induced in an excited-pole winding by a single sweep oroscillation of the armature past about the commanded position has such awaveform that the magnitude of the induced voltage is zero at thecommanded position even though the velocity of the armature is a maximumat this point. A dampingforce for an oscillating mass should begenerally proportional to the velocity of the mass and have a maximumvalue when the velocity is at a maximum. Therefore, the voltages inducedin the excited pole windings for both uni-polar and bi-polar excitationschemes are not suitable for damping control signals because thevoltages do not vary in a proper manner with the armature velocity.

However, if the voltages induced in windings other than those which aremomentarily excited in the unipolar and bi-polar excitation schemes areexamined, a different relationship between the induced voltages and thearmature velocity is found. The teeth of each nonexcited pole areasymmetrically offset from the armature teeth or midway between thealigned and symmetrically offset positions and therefore the fluxpassing through such non-excited pole varies about a point on a fluxversus displacement curve which is offset from the commanded or neutralposition and which in FIG. 3 may be taken as either the point X or Xdepending on the particular non-excited pole examined. By inspection, itis apparent that the non-excited poles have induced in their windings,as a result of the oscillatory overshoot of the armature, voltagewaveforms which are approximately proportional to the velocity of thearmature since the flux versus displacement curve in the vicinity of Xor X is substantially linear. Accordingly, the set of poles havingasymmetrically offset teeth relative to the armature teeth in any givenstep of the excitation sequence are appropriate sources of an electricaldamping signal for attenuating the oscillations of the armature 12 atits commanded position.

Another important feature of the present invention resides in the forcegenerating means for imposing a motion-opposing or damping force onthemotor armature. The force generating means responds to the signalsproduced by the sensing means and also utilizes a set of motor poleshaving asymmetrically offset teeth during the given step of theexcitation sequence in question to damp the armature movements byimposing a magnetomotive damping force on the armature.

Since the motor poles assume different positional relationships with thearmature teeth during the different steps of the excitation sequence,the poles used for sensing and for applying damping forces are differentfor each step and the damping system must include some switching deviceor devices for connecting other parts thereof to the proper windings ateach step position of the motor. In particular, the system includes afirst switching or input selector means causing the inducedelectromotive forces in the proper windings to be detected atappropriate times and a second switching means or output selector whichprovides reversible current to the proper windings to generate amagnetomotive damping force. One such system is shown in the diagram ofFIG. 4.

In FIG. 4 a conventional driver logic 40 and a unit 42, consisting of aset of driver switches and power amplifiers, are operatively connectedwith the four poles of the step motor 10, shown schematically. Thedetails of the construction of the driver logic and of the driverswitches and amplifiers are not shown since they form through the cable46, to the driver switches of the unit 42 to cause the windings of themotor to be energized in the proper manner. The detailed construction ofa step motor driver having conventional driver logic and driver switchesof the type suitable for controlling the step motor in the presentinvention is shown in the US. Pat. No. 3,466,520 to Aylikci etal.'issued Sept. 9, 1969.

In the embodiment of the invention in FIG. 4, the magnetomotive dampingforce on the motor armature is generated by an asymmetric motor poledifferent from that employed to detect the velocity signal. As usedherein, and as previously explained, the term asymmetric motor pole isused to refer to a motor pole which at 'the moment in question is notfully aligned or symmetrically arranged relative to the armature teeth,assuming the armature to be at its commanded position. In order to sensean EMF representative of the armature velocity, each of the motorwindings is connected toan input selector circuit 50. The input selectorcircuit 50 is basically a switching device controlled synchronously withthe stepping motion of the motor by signals from the driver logic 40transmitted to it through a cable 56. More specifically, the inputselector circuit 50 is phased with the stepping motions of the motorarmature so that it sequentially connects the motor windings to theinput of the damping amplifier 52in such a manner that for each stepposition of the motor only one winding is connected to the amplifier 52,such one winding being the winding of an asymmetric motorpole. Theconstruction of the selector circuit 50 is similar to the constructionof the driver switching unit 42 in analogue form since the switchingoccurs synchronously with that of the driver switches to transmitanalogue voltages from theasymmetric motor pole windings. The dampingamplifier 52 responds to thesignal induced in the motor windingconnected to it by the input selector circuit 50 and as a resultproduces an output current signal which is supplied through an outputselector circuit 54 to another one or more of the asymmetric motor polewindings to generate a magnetomotive force opposing the armatureoscillation. The output selector circuit 54 is or may be a switchingdevice generally similar to the input selector controlled by signalsissuing from the driver logic 40 through the cable 56 in phasedrelationship with the stepping motion of the motor 10 tosequentiallyconnect the output of the amplifier 52 to the motor windings in suchamanner that for each step position of the motor the output of theamplifier is connected to the winding of at least one asymmetric poleother than the one winding used to produce the input signal. In a motorhaving a large number of asymmetric poles for each step position theamplifier output is preferably connected to the windings of a number ofsuch poles.

For example, if the motor poles and armature are positioned as'shown inFIG. 2, the input selector circuit 50 second set of input lines 73, 73which are sequentially would operate to sense the voltage induced in thewinding 26b and the output selector circuit 54 would operate to energizethe winding 26:! with the output of the amplifier 52.

FIG. 5 discloses another embodiment of the invention which differs fromthat of FIG. 4 in that it derives the damping control or input signalfrom the same asymmetric motor pole winding as employed to generate themagnetomotive damping force on the armature. In this embodiment, thedriver logic and the driver switch and amplifier unit 42 are or may bethe same as those disclosed in FIG. 4. The stepping motor,

generally designated 60, has a construction slightly different from theconstruction of the motor 10 of FIG. 4 in that dropping resistors 64, 64are connected between the respective motor windings 62, 62 and ground.The input selector circuit 70 has one set of input lines 71, 71 whichare sequentially connected, one at a time, through the circuit 70 to acorresponding output line 75 containing a dummy inductor 72 and a dummyresistor 74 connected in series with one another and which haveimpedance values approximately ten times larger than the individualwindings 62, 62 and resistors 64, 64, respectively. At its other endeachinput line 71 is connected to one end of an associated one of the motorwindings and to the output of the driver switch associated with eachwinding. The circuit 70 also has a connected, one at a time, through thecircuit 70 to a corresponding output line 77 leading directly to adifferential damping amplifier 76. At its other end each input line 73is connected to the junction between an associated one of the motorwindings and its dropping resistor 64. The motor windings 62, 62 and theresistors 64, 64 are sequentially placed in parallel with the dummyinductor 72 and the dummy resistor 74 through the first set of inputlines 71, 71 and the corresponding output line 75 to sequentially formbridge circuits. The inductor 72 and the resistor 74 comprise the highimpedance leg of each sequential bridge circuit and the windings 62, 62and the resistors 64, 64, taken in sequence comprise the low impedancelegs of the bridge circuits. The sequencing through the second set ofinput lines 73, 73 by the input selector circuit is correlated with thesequencing through the'first set of input lines 71, 71 so that thebalance voltages across each sequentially formed bridge circuit appearsat the inputs of the differential amplifier 76. I

The output of the amplifier 76 is fed through the output selectorcircuit 78 to the respective windings 62, 62 over output lines 79, 79.Again, the output selector circuit 78 and the input selector circuit 70.are synchronous switching devices, such as selector circuits 50 and 54in FIG 4 and are controlled in phased relationship with the stepping.rate of the motor by. the signals from the driver logic 40 suppliedthroughthe cable 56; however, the synchronization is such that thedamping signal is detected in the same asymmetric motor pole winding asemployed to generate the damping force. In other words, the input line71 which during any given step position is used to supply the input ordamping control signal is connected to the same motor pole winding asthe output line 79 on which the output of the output selector 78appears. While separate dropping resistors 64, 64 are shown incorporatedin the motor 60, a single grounded resistor connected to the input line77 of amplifier 76 would serve the same function provided that the motorwindings are not grounded.

The bridge circuits sequentially formed by the input selector circuit 70utilize the dummy inductor 72 and the dummy resistor 74 to simulate themotor pole windings in the higher impedance leg of the bridge. When novoltage is induced in the asymmetric pole windings by the motorarmature, which indicates that the armature is stationary, the bridge isbalanced and in a null condition. If the armature is moving, however,the voltage induced in the pole winding of the bridge unbalances thebridge and a reversible damping current is supplied from the amplifier76 to the same pole winding by the output selector circuit 78 togenerate the magnetomotive damping force on the armature.

While the novel step motor damping system has been described in a numberof embodiments, it will be understood that numerous modifications andsubstitutions can be made without departing from the spirit of theinvention. For example, where the step motor incorporates more than oneset of windings excited in a phased sequence, the damping apparatus neednot utilize all of the sets for damping nor is it necessary that thenumber of specific sets of windings used for sensing be the same as thenumber of specific sets of windings used for imposing the damping forceon the armature. It may also be desirable to incorporatea speed sensingcutout to eliminate the damping force on the armature during high-speedmotion of the armaturenSuch a cutout may be a pulse rate sensorconnected to the driver logic 40 and a switch for interrupting thedamping amplifier output. It should also be understood that the conceptof employing motor pole windings to sense a velocity signal and impose adamping force on the motor armature extends to fiat or linear stepmotors such as the type, for example, in which the motor armature is anelectromagnetic head or forcer which is translated on a fluid cushionover a magnetically permeable platen having a waffle iron arrangement ofteeth by exciting of pole windings of the electromagnetic forcer in aphased electrical excitation cycle. Accordingly, the present inventionis disclosed in several embodiments merely by way of illustration ratherthan limitation. I

I claim:

1. In a step motor drive system having a controller which excites thewindings of the motor poles in a phased electrical excitation cycle toproduce both excited and non-excited motor poles in each step of thephased electrical excitation cycle and to thereby produce motive forcesresulting in a stepping motion of the motor armature, the improvementcomprising: sensing means incorporating the windings of the motor poleswhich produce the motive forcesfor detecting movement of the motorarmature through non-excited windings in each phase of the electricalexcitation cycle and producing a damping signal; and force generatingmeans including the motor pole windings for imposing through thenon-excited windings a damping force on the motor armature in responseto the damping signal produced by said sensing means.

2. An improvement as defined in claim 1 wherein said sensing meansfurther includes first sequencing for sequentially detectingelectromotive forces induced in the respective pole windings by movementof the motor armature and for sequentially providing the detectedelectromotive forces at the output.

3. An improvement as defined in claim 2 wherein said first sequencingmeans is a synchronous switching means connected to the controller andhaving a switching rate synchronized-by the controller with the steppingmotion of the armature.

4. An improvement as defined in claim 3 wherein said synchronousswitching means has a phase relationship with the stepping motion of thearmature which sequentially associates the output of said synchronousswitching means with the inputs of said synchronous switching meansassociated with selected non-excited motor pole windings in the phasedelectrical excitation cycle.

5. An improvement as defined in claim 3 in the drive system for a stepmotor having teeth on both the armature and the stationary motor poleswhich teeth are pitched to cyclically assume symmetric and asymmetricpositional relationships with one another as the armature moves relativeto the stationary motor poles during the phased electric excitationcycle wherein said synchronous switching means has a phase relationshipwith the stepping motion of the armature which sequentially associatesthe-output of said synchronous switching means with the inputs of saidsynchronous switching means associated with the windings of selectedmotor poles assuming the asymmetric positional relationship in thephased electrical excitation cycle.

6. An improvement as defined in claim Z wherein the force generatingmeans includes reversible current driver means connected to the signaltransmitting output of said first sequencing means and with the motorpole windings for supplying the motor pole windings with a reversiblecurrent in response to the induced electromotive forces detected by saidfirst sequencing means.

7. An improvement as defined in claim 6 wherein said reversible currentdriver means in the force generating means includes asignal amplifierand second sequencing means having an input connected with the amplifierand outputs connected respectively with the motor pole windings forsequentially imposing the amplified signal on the windings and whereinsaid first and second sequencing means are synchronous switching meansconnected to the controller of the drive system to synchronize thesequencing of the first and second sequencing means with the phasedexcitation of the motor pole windings by the controller.

8. An improvement as defined in claim 6 in the driving system having acontroller which produces at least two non-excited windings during eachphase of a bipolar electrical excitation cycle wherein said firstsequencing means is a synchronous switching means connected to thecontroller and having a switching rate phased by the controller with thestepping motion of the armature for connecting the output of said firstsequencing means with an input of said first sequencing means associatedwith one of the non-excited motor pole windings and said driver means isphased by the controller with the stepping motion of the armature forsupplying current to the other of the non-excited motor pole windings.

' 9. An improvement as defined in claim 6 in the driving system having acontroller which produces at least two non-excited windings during eachphase of a bipolar electrical excitation cycle wherein said firstsequencing means is a synchronous switching means connected to thecontroller and having a switching rate phased by the controller with thestepping motion of the armature for connecting the output of said firstsequencing means with an input of said first sequencing means associatedwith a non-excited motor pole wind-.

ing and said driver means is phased by the controller with the steppingmotion of the armature for supplying current to the same non-excitedmotor pole winding.

10. An improvement as defined in claim 1 wherein said force generatingmeans further includes synchronous switching means connected to themotor pole windings and the controller and having a switching ratesynchronized by the controller with the stepping motion of the armaturefor sequentially switching the damping signal between motor polewindings in synchronism with the stepping motion of the armature.

11. The method of damping movements of a step motor armature where themovements are produced by sequentially exciting the windings of themotor poles in a phased electrical. excitation. cycle comprising thesteps of sequentially sensing theelectromotive forces induced inselected non-excitedmotor pole windings by the armature movements insynchronism with the sequential excitation of, the phased electricalexcitation cycle and; sequentially imposing a damping force. on thearmature through selected non-excited motor poles in response to thesensed electromotive forces.

12. A method of damping as defined in claim 11 where during each phaseof the electrical excitation cycle the armature movements induceelectromotive forces uniquely associated with the armature velocity v inone set of the non-excited motor pole windings and induce electromotiveforces not uniquely associated with the armature velocity in'another setof the nonexcited motor pole windings wherein the step of sensingfurther comprises sensing the induced electromotive forces uniquelyassociated with the armature velocity in said one set of the non-excitedmotorpole windings during one phase of the electrical excitation cycle.

13. A method of damping as defined in claim 11 wherein the step ofimposing comprises sequentially energizing the selected non-excitedmotor pole windings in synchronism with the phased electrical excitationcycle to generate a magnetomotive' damping force on the motor armature.

14. A method of damping as defined in claim 11 in which method themovements of the motor armature at a commanded position of the armatureinduce electromotive forces having a zero value in a first set of thenon-excited motor pole windings and electromotive forceshaving anon-zero value in a second set of the non-excited motor pole windingswherein the step of sensing comprises sequentially sensing the inducedelectromotive forces in the non-excited motor pole windings of thesecond set; and the step of imposing comprises sequentially energizinthe non-excited motor pole windings of the secon set in response to theinduced electromotive forces sensed in the motor pole windings of thesecond set. I

15. The method of damping oscillations of a magnetically polarized motorarmature in a step motor as the ing signal.

1. In a step motor drive system having a controller which excites thewindings of the motor poles in a phased electrical excitation cycle toproduce both excited and non-excited motor poles in each step of thephased electrical excitation cycle and to thereby produce motive forcesresulting in a stepping motion of the motor armature, the improvementcomprising: sensing means incorporating the windings of the motor poleswhich produce the motive forces for detecting movement of the motorarmature through non-excited windings in each phase of the electricalexcitation cycle and producing a damping signal; and force generatingmeans including the motor pole windings for imposing through thenon-excited windings a damping force on the motor armature in responseto the damping signal produced by said sensing means.
 2. An improvementas defined in claim 1 wherein said sensing means further includes firstsequencing means having inputs connected respectively with the motorpole windings and a signal transmitting output for sequentiallydetecting electromotive forces induced in the respective pole windingsby movement of the motor armature and for sequentially providing thedetected electromotive forces at the output.
 3. An improvement asdefined in claim 2 wherein said first sequencing means is a synchronousswitching means connected to the controLler and having a switching ratesynchronized by the controller with the stepping motion of the armature.4. An improvement as defined in claim 3 wherein said synchronousswitching means has a phase relationship with the stepping motion of thearmature which sequentially associates the output of said synchronousswitching means with the inputs of said synchronous switching meansassociated with selected non-excited motor pole windings in the phasedelectrical excitation cycle.
 5. An improvement as defined in claim 3 inthe drive system for a step motor having teeth on both the armature andthe stationary motor poles which teeth are pitched to cyclically assumesymmetric and asymmetric positional relationships with one another asthe armature moves relative to the stationary motor poles during thephased electric excitation cycle wherein said synchronous switchingmeans has a phase relationship with the stepping motion of the armaturewhich sequentially associates the output of said synchronous switchingmeans with the inputs of said synchronous switching means associatedwith the windings of selected motor poles assuming the asymmetricpositional relationship in the phased electrical excitation cycle.
 6. Animprovement as defined in claim 2 wherein the force generating meansincludes reversible current driver means connected to the signaltransmitting output of said first sequencing means and with the motorpole windings for supplying the motor pole windings with a reversiblecurrent in response to the induced electromotive forces detected by saidfirst sequencing means.
 7. An improvement as defined in claim 6 whereinsaid reversible current driver means in the force generating meansincludes a signal amplifier and second sequencing means having an inputconnected with the amplifier and outputs connected respectively with themotor pole windings for sequentially imposing the amplified signal onthe windings and wherein said first and second sequencing means aresynchronous switching means connected to the controller of the drivesystem to synchronize the sequencing of the first and second sequencingmeans with the phased excitation of the motor pole windings by thecontroller.
 8. An improvement as defined in claim 6 in the drivingsystem having a controller which produces at least two non-excitedwindings during each phase of a bi-polar electrical excitation cyclewherein said first sequencing means is a synchronous switching meansconnected to the controller and having a switching rate phased by thecontroller with the stepping motion of the armature for connecting theoutput of said first sequencing means with an input of said firstsequencing means associated with one of the non-excited motor polewindings and said driver means is phased by the controller with thestepping motion of the armature for supplying current to the other ofthe non-excited motor pole windings.
 9. An improvement as defined inclaim 6 in the driving system having a controller which produces atleast two non-excited windings during each phase of a bi-polarelectrical excitation cycle wherein said first sequencing means is asynchronous switching means connected to the controller and having aswitching rate phased by the controller with the stepping motion of thearmature for connecting the output of said first sequencing means withan input of said first sequencing means associated with a non-excitedmotor pole winding and said driver means is phased by the controllerwith the stepping motion of the armature for supplying current to thesame non-excited motor pole winding.
 10. An improvement as defined inclaim 1 wherein said force generating means further includes synchronousswitching means connected to the motor pole windings and the controllerand having a switching rate synchronized by the controller with thestepping motion of the armature for sequentially switching the dampingsignal between motor pole windings in synchronism with the steppingmotion of the armature.
 11. The method of damping movements of a stepmotor armature where the movements are produced by sequentially excitingthe windings of the motor poles in a phased electrical excitation cyclecomprising the steps of sequentially sensing the electromotive forcesinduced in selected non-excited motor pole windings by the armaturemovements in synchronism with the sequential excitation of the phasedelectrical excitation cycle and; sequentially imposing a damping forceon the armature through selected non-excited motor poles in response tothe sensed electromotive forces.
 12. A method of damping as defined inclaim 11 where during each phase of the electrical excitation cycle thearmature movements induce electromotive forces uniquely associated withthe armature velocity in one set of the non-excited motor pole windingsand induce electromotive forces not uniquely associated with thearmature velocity in another set of the non-excited motor pole windingswherein the step of sensing further comprises sensing the inducedelectromotive forces uniquely associated with the armature velocity insaid one set of the non-excited motor pole windings during one phase ofthe electrical excitation cycle.
 13. A method of damping as defined inclaim 11 wherein the step of imposing comprises sequentially energizingthe selected non-excited motor pole windings in synchronism with thephased electrical excitation cycle to generate a magnetomotive dampingforce on the motor armature.
 14. A method of damping as defined in claim11 in which method the movements of the motor armature at a commandedposition of the armature induce electromotive forces having a zero valuein a first set of the non-excited motor pole windings and electromotiveforces having a non-zero value in a second set of the non-excited motorpole windings wherein the step of sensing comprises sequentially sensingthe induced electromotive forces in the non-excited motor pole windingsof the second set; and the step of imposing comprises sequentiallyenergizing the non-excited motor pole windings of the second set inresponse to the induced electromotive forces sensed in the motor polewindings of the second set.
 15. The method of damping oscillations of amagnetically polarized motor armature in a step motor as the armature ismoved in a stepped manner between commanded positions by excitation of aset of motor pole windings in a phased sequence comprising sensingreversible electromotive forces induced in a non-excited motor polewinding of the set by the armature movements about a commanded positionduring one phase of the excitation sequence; generating a reversibledamping signal in response to the sensed electromotive forces; andenergizing a non-excited motor pole winding of the set during the onephase of the excitation sequence in accordance with the reversibledamping signal.